1. Field of the Invention
This invention relates to a cutting instrument, and more particularly to a gear-forming cutter that machines a non-metallic workpiece to form gear teeth on the same.
2. Description of the Related Art
Common gear-forming methods include a rolling process (see FIG. 1) and a milling process (see FIG. 2). Referring to FIG. 1, in the rolling process, a tooth-rolling cutter 2 is rotated to machine a metallic blank 1. The tooth-rolling cutter 2 rotates about a rotating axis perpendicular to that of the metallic blank 1. When the metallic blank 1 and the tooth-rolling cutter 2 rotate at an appropriate speed ratio, the tooth-rolling cutter 2 is brought into press contact with the metallic blank 1 to form a plurality of gear teeth on the same. Referring to FIG. 2, in the milling process, a milling cutter 3 is disposed parallel to a metallic blank 1 to machine the same. When the metallic blank 1 and the milling cutter 3 rotate in opposite directions, a plurality of gear teeth are formed on the metallic blank 1.
Although the tooth-rolling cutter 2 and the milling cutter 3 can form the gear teeth on the metallic blank 1, these machining devices are bulky. Therefore, the tooth-rolling cutter 2 and the milling cutter 3 are not suitable for machining non-metallic precision instruments, e.g., a barrel 4 (see FIG. 3) of a zoom lens of a camera. Referring to FIG. 3, in order to enable the barrel 4 to be driven, the barrel 4 includes a toothed driving ring 400 that has an annular outer surface, which is formed with axial teeth 401 and helical teeth 402. Because of the small size of the barrel 4 and the inclusion of both the axial teeth 401 and the helical teeth 402 on the driving ring 400, the driving ring 400 cannot be formed by means of the tooth-rolling cutter 2 and the milling cutter 3.